Grant Bigg and Amy Kirbyshire of the department of geography at Sheffield University examined temperature records of central England over recent decades, together with observations of 140 types of summer flowering plant, such as geraniums and roses, and when they came into bloom.
To determine the onset of summer, they looked for the third day of each year when average temperatures reached 14C. That may sound distinctly chilly for summer, but comfortably allows for daytime temperatures above 20C.
“We wondered if we could set a defining moment of when summer begins,” Bigg said.
According to the analysis, summer should, on average, arrive in Britain tomorrow.
Records show that in the period 1954-1963, the average date for the third such day was 25 May. By the 1990s, it had shifted forwards to 14 May. By 1998-2007, on average, summer arrived on 7 May. The shift is consistent with global warming, Bigg said. “It’s always very difficult to make direct attributions but scientists say global warming is very likely driven by human activity and I think we can say the same thing.” The researchers saw a similar, though smaller, pattern with summer plant flowering. On average, the first flowering date for 1954-1963 was 29 May. By 1991-2000 it was 26 May.
Announcing their results in the journal Climatic Change, the duo say they “present a convincing argument that the onset of the British summer season has become increasingly early in the last 50 years”. The finding is consistent with similar studies that have used the timing of natural events to investigate the onset of spring and autumn.
Earlier summers may have encouraged drought or heatwave conditions by prolonging the period of warm temperatures, the scientists suggest. The earliest recorded summer onset day was 18 April in 2003, which was followed by a record breaking heat wave.
An early start does not always herald a good summer. The second-earliest onset day was in 2007, which preceded the wettest summer in England and Wales since records began in 1766. “An early summer onset is clearly no guarantee of a barbecue summer,” the scientists say.
This year is not following the early summer pattern however, as there has not yet been a day with an average temperature of 14C.